CMOS image sensors are increasingly being used as low cost imaging devices over Charge Coupled Device (CCD) image sensors. A conventional CMOS image sensor circuit includes a focal plane array of pixel cells, each one of the cells includes a photo-conversion device for generating charge in response to light incident on the pixel cell. Each pixel cell typically includes a transistor for transferring charge from the photo-conversion device to a sensing node, and a transistor, for resetting a sensing node to a predetermined charge level prior to charge transference. The pixel cell also typically includes a source follower transistor for receiving and amplifying charge from the sensing node and an access transistor for controlling the readout of the cell contents from the source follower transistor.
In a conventional CMOS image sensor, the photo-conversion device converts photons to charge and accumulates the photo-generated charge, while other active elements of the pixel cells, such as transistors, amplify the charge.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994). See also U.S. Pat. Nos. 6,177,333 and 6,204,524, which describe operation of conventional CMOS image sensors, the contents of which are incorporated herein by reference.
The basic structure of a photodiode conventionally used in CMOS image sensors includes a p-n junction typically formed as doped regions in a silicon substrate, wherein the p-n junction is operated under a reverse-biased electrical field. The photon to charge conversion occurs as the photons are absorbed into the photodiode and the energy from the light induces ionization in the depletion region at the p-n junction by causing the electrons of the ionized atoms to move from the valence band to the conduction band, leaving behind a hole. The amount of charge generated is proportional to the intensity of the incident light on the photodiode.
Other optoelectronic devices use different types of photodiodes. For example, avalanche photodiodes have been used in light detectors to convert incident light into electrical charge. In addition to photon-to-charge conversion, avalanche photodiodes amplify the photo-generated charge. An example of an avalanche photodiode (APD) is shown in FIG. 1, and includes a light absorbing layer 2, a multiplication layer 4, and a substrate 6. Electrodes 8 and 10 are placed in contact with the light absorbing layer 2 and the substrate 6, respectively, and a bias source 12 connected between the electrodes 8, 10 to create an electric field through the entire APD structure. The layers of the APD are typically materials which consist of Group III and V elements of the periodic table.
Light 14 is directed onto the light absorbing layer 2 and the energy from the photons generates the initial charge carriers by creating a number of electron-hole pairs in the previously neutral atoms of the material of the light absorbing layer. The initially created carriers, electrons and holes, are moved by the electric field into the multiplication layer 4. The holes and electrons generated by the incident light may gain energy as they move to layer 4. When a hole or electron has acquired sufficient energy it may subsequently undergo impact ionization collisions and create a second generation of electrons and holes. Additionally, carriers of the first generation as well as subsequent generations, may then gain energy to ionize, generating more carriers, and so forth. In this manner, the electrical charge generated by the incident light is amplified.
The secondary and subsequent generations of charge carriers may acquire sufficient energy to impact ionize by drifting through the material in the presence of the electric field, as described by R. J. McIntyre, “A New Look at Impact Ionization—Part I: A Theory of Gain, Noise, Breakdown Probability, and Frequency Response,” IEEE Transactions on Electron Devices, vol. 46, No. 8, August 1999, the disclosure of which is hereby incorporated by reference.
Avalanche photodiodes amplify charge, but the amplification process may also produce significant noise, typically referred to as the excess noise factor F. In the well recognized article entitled “Multiplication Noise in Uniform Avalanche Diodes,” IEEE Transactions on Electron Devices, vol. 13, pp. 164-168, 1966, which is incorporated herein by reference, R. J. McIntyre demonstrates that the excess noise factor can be minimized by maximizing the number of ionizing collisions by one type of carrier and minimizing impact ionizations by the other.
The excess noise factor, F, can be described by the following equation:F(M)=kM+(2−1/M)(1−k),where M is the current multiplication factor, and k is the ratio of a first carrier type ionization rate to the second carrier type ionization rate. In the presence of a relatively low electric field, k is a function of the magnitude of the electric field and the rate of change of the electric field. The higher the electric field in a region that a carrier has moved from, the more likely it is that the carrier will ionize.
It would be advantageous to provide a CMOS image sensor that includes a photo-conversion device capable of charge amplification and with reduced noise for minimized dark current.